For example here in Tasmania at the present tariffs if I run my 2400W (2.4kW) for 5 hours of sewing it is going to cost me 2.4 x 5 x 0.268 = $3.21
If you have a smaller travel sized iron or class iron have a look at its power rating and you will more than likely find that it uses up to 2/3 less electricity than your normal iron. Mine only uses 800W of power per hour.
This iron will cost me 0.8 x 5 x .268= $1.07
Definitely worth thinking about how you use that hot rock in your sewing room. More money saved means more fabric purchased and that has to be good thing.